Satellite positioning system receivers and methods

ABSTRACT

A method in a satellite positioning system receiver having stored almanac data including determining information for a satellite using ephemeris data ( 710 ), determining information for the same satellite using the stored almanac data ( 722 ), determining an error between the satellite information determined from the ephemeris data and the satellite information determined from the stored almanac data ( 730 ), and updating the stored almanac data based upon the error ( 734 ).

FIELD OF THE DISCLOSURE

[0001] The present disclosure relates generally to satellite positioningsystem (SPS) receivers, and more particularly to acquiring satelliteinformation used for approximating the initial position of and locatingSPS receivers, for example, Global Positioning System (GPS) enabledmobile wireless communications subscriber devices, and methods.

BACKGROUND OF THE DISCLOSURE

[0002] The Global Positioning System (GPS) is a satellite based locationand time transfer system developed by the United States government andavailable free of charge to all users. Other satellite positioningsystems (SPS) have also been or are being developed, including Glonasssatellite system in Russia and the Galileo system in Europe.

[0003] The location of an SPS receiver is based upon a one-way rangingbetween the SPS receiver and several satellites, which transmit signalshaving the times-of-transmission and orbital parameters for theirrespective time variable locations-in-space. An SPS receiver acquiressatellite signals by correlating internal replica signals to carrierfrequencies and distinguishable codes for each of several in-viewsatellites. When satellite signals have been acquired, the SPS receiveruses time and the orbital parameter information from the acquiredsatellites for measuring ranges to the satellites, preferably four ormore satellites. These measured ranges are called pseudoranges becausethey include a term caused by a time error of the SPS receiver clock.

[0004] The SPS satellite pseudoranges are measured by determining phaseoffsets between pseudorandom (PRN) codes of the received satellitesignals and the internal replica PRN codes referenced to the SPSreceiver clock. Some SPS receivers measure and integrate the carrierphases of the satellite signals in order to reduce noise on the measuredphase offsets. The SPS receiver then determines an SPS-based time bymonitoring the SPS signals until a TOW field is decoded. The SPS-basedtime is used to determine the times that the phase offsets weremeasured. The measurement times are then used with ephemeris datareceived from the satellites for calculating instantaneouslocations-in-space of several satellites and for linearizing locationequations relating the calculated locations-in-space to the measuredpseudoranges. Having four or more linearized location equations for fouror more satellites, respectively, SPS receivers can resolve their3-dimensional geographical location and correct the time error in theirinternal clocks.

[0005] It is known generally to use almanac and ephemeris informationstored on SPS receivers to speed the acquisition of satellites. Thealmanac data contains coefficients to Kepler's equations of satellitemotion and is useful for computing which satellites are visible at aparticular time. The almanac data may also be used for computingsatellite location and velocity vectors, from which satellite Dopplerestimates may be computed for aiding signal acquisition. The almanacdata provides low-resolution satellite position accuracy, which istypically no better than about 1 kilometer when fresh. Almanac datahowever contains a relatively small number of bytes, approximately 1200bytes for 32 satellites, and almanac data is useful for 6 months to 1year depending on whether satellites are re-positioned or new satelliteshave been added or removed from the constellation. In the GPSconstellation, each satellite broadcasts almanac data, which is updatedevery few days, for all GPS satellites on a twelve and one-half minutecycle.

[0006] Ephemeris data is similar to almanac data but provides far moreaccurate satellite position information, which is accurate to withinseveral meters if the ephemeris data is not more than a few hours old.The accuracy of satellite position information derived from ephemerisdata degrades with time. SPS receivers typically use ephemeris data forcomputing precise satellite locations, which may be used for positioncomputation when combined with SPS receiver measured pseudorangeinformation. A GPS constellation ephemeris data set for one satellite isapproximately 72 bytes of data, and thus ephemeris data for all 32 GPSsatellites requires about 2304 bytes of data storage space. In the GPSconstellation, each satellite broadcasts its own ephemeris data everythirty-seconds. An SPS receiver must acquire a satellite in order toobtain its ephemeris data.

[0007] In a typical GPS receiver, for example, in GPS enabled cellularcommunications and stand-alone navigation devices, the time to acquirenew almanac data directly from a satellite requires more than twelve anda half minutes (12.5 minutes). Operating GPS receivers for therelatively long period required to obtain almanac data directly from asatellite draws substantially charge from the battery, which isundesirable in many applications including GPS enabled cellulartelephones. The time required to obtain ephemeris data in this manner iscomparatively small, at approximately thirty seconds (30 sec.).

[0008] It is known to provide almanac information to GPS enabled radiocommunications devices in an over-the-air radio message, as disclosed,for example, in U.S. Pat. No. 6,064,336 entitled “GPS Receiver UtilizingA Communication Link”, among other patents and publications. In someinstances, however, it is undesirable to use almanac information or toobtain it in an over-the-air message.

[0009] It is also known to provide ephemeris information to GPS enabledradio communications devices in an over-the-air radio message, asperformed, for example, by the Motorola Eagle GPS receivers. In priorart FIG. 1, GPS satellite ephemeris and almanac information 10 istransmitted from a cellular communications network base-station 12 to awireless subscriber device 14 using an over-the-air communicationsprotocol. Wireless subscriber device 14 contains a GPS receiver 16 withan antenna, a cellular transceiver 20, and two databases, stored inmemory, to store ephemeris data 22 and almanac data 24. The GPS receiver16 can acquire both almanac and ephemeris data directly from GPSsatellites via antenna 18 and store them into the almanac database 24and ephemeris database 22. In addition, the cellular transceiver 20 canacquire fresh almanac and ephemeris data 10 from the cellular networkvia over-the-air messages.

[0010] Transmitting satellite almanac and ephemeris data over acommunications link however requires costly network infrastructure.Additionally, relatively long data strings are required for thetransmission of ephemeris and almanac data, and the management ofrequesting and storing the data derived from over-the-air messages iscumbersome. Other GPS receiver applications, including vehiclenavigation, do not include a radio, which could be used for receivingover-the-air assistance messages. For these and other reasons, in atleast some applications, it is undesirable to obtain almanac data fromover-the-air assistance messages.

[0011] U.S. Pat. No. 6,437,735 entitled “Position Detection SystemIntegrated into Mobile Terminal” discloses receiving ephemeris data at amobile GPS receiver either directly from GPS satellites or from awireless communications network, and transforms the ephemeris data toalmanac information by scaling and masking ephemeris parameters to formcorresponding almanac parameters, which are stored on the GPS receiverfor positioning determination. Almanac data derived in this manner isbelieved have substantial errors, for example, accumulated error in thealong-track direction, which will likely produce unacceptable resultsover very long time periods.

[0012] The various aspects, features and advantages of the disclosurewill become more fully apparent to those having ordinary skill in theart upon careful consideration of the following Detailed Descriptionthereof with the accompanying drawings described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is prior art system architecture for communicating GPSsatellite almanac and ephemeris information from networks to subscriberdevices.

[0014]FIG. 2 is a schematic block diagram of an exemplary GPS receiver.

[0015]FIG. 3 is a process flow diagram.

[0016]FIG. 4 is another schematic block diagram of an exemplary GPSreceiver.

[0017]FIG. 5 is an exemplary signal having information frames.

[0018]FIG. 6 is schematic illustration of a process for derivingsatellite orbital information from ephemeris information.

[0019]FIG. 7 is a schematic illustration of a process for updatingalmanac information with ephemeris information.

[0020]FIG. 8 is a plot of satellite position vector differences betweenalmanac-derived satellite positions and ephemeris-derived satellitepositions, as a function of almanac and ephemeris age.

[0021]FIG. 9 is a plot of satellite velocity vector differences betweenalmanac-derived satellite velocities and ephemeris-derived satellitevelocities, as a function of almanac and ephemeris age.

[0022]FIG. 10 is a plot of satellite position vector differences betweenalmanac-derived satellite positions and ephemeris-derived satellitepositions, for the case of fresh ephemeris and an almanac that is 40days old, as a function of ephemeris age in days.

[0023]FIG. 11 is a plot of satellite velocity vector differences betweenalmanac-derived satellite velocities and ephemeris-derived satellitevelocities, for the case of fresh ephemeris and an almanac that is 40days old, as a function of ephemeris age in days.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0024] According to one aspect of the disclosure, a satellitepositioning system (SPS) receiver automatically acquires or attempts toacquire almanac data any time the SPS receiver is connected to externalpower, for example, to a battery charger or a car-kit adaptor. In thismode of operation, the SPS receiver continuously operates the GPSreceiver to demodulate almanac data or other information, for example,from signals received directly from a satellite or from some othersource, while connected to the external power supply.

[0025] In FIG. 2, a GPS receiver 210 in a cellular handset 220 acquiressatellite navigation data directly from a GPS satellite 230 via GPSantenna 232. Once received, the satellite navigation data, for example,almanac and/or ephemeris data, is stored in memory, for example, inephemeris database 234 and almanac database 236. The GPS receiver 210 iscontinuously powered ON and attempting to demodulate ephemeris andalmanac data directly from GPS satellites 230 as long as power isapplied to the handset externally via an external charger connector 240.The external power may be from a battery charger, a cigarette lighteradapter, a hands-free car adaptor, or similar external power supply thatsupplies power to the entire phone instead of from the batteriesinternal to the SPS receiver.

[0026] In the process diagram 300 of FIG. 3, at block 310, adetermination is made whether the SPS receiver, for example, embedded ina cellular subscriber device, is connected to a power supply other thanits internal battery. At block 320, satellite navigation information isreceived if the receiver is coupled to the external power supply. Thesatellite information, for example, ephemeris and/or almanac data, maybe received directly from a satellite or alternatively from some othersource without regard to battery power consumption since the receiverdoes not operate on battery power. At block 330, the satellitenavigation information is stored on the receiver, for example, inmemory.

[0027] In one embodiment, reception of satellite positioning systemnavigation data begins when the receiver is connected to a power supplyother than its battery. Generally, the reception of satellitepositioning system navigation data is discontinued if the receiver isreconnected to its battery. In some embodiments, however, it may bedesirable to continue reception of the SPS navigation data uponreconnecting the SPS receiver to its battery until reception of thenavigation data is complete. In some embodiment, navigation data, forexample almanac data, is downloaded directly from the satellite only ifthe receiver is coupled to external power.

[0028] In FIG. 3, at block 340, a determination is made whether batterypower has been re-connected, for example, upon disconnecting theexternal power source. If the download of satellite data is incompleteat block 350, a determination is made at block 360 whether a conditionis satisfied that would require or justify completion of the downloadunder battery power. The condition assessed at block 360 may be that thedownloading data is essential or that the download is nearly complete,which may be determined, for example, by assessing whether apredetermined portion, or percentage, of the navigation data has alreadybeen received. In FIG. 3, at block 370, the download is ended if thecondition is not satisfied, and at block 380 the download is completedif the condition is satisfied.

[0029] According to another aspect of the disclosure, an SPS receiver isoperated synchronously with an expected time of arrival of information,for example, satellite almanac and/or ephemeris information. Thusoperated, the SPS receiver does not remain idle and consume power whennot receiving information. FIG. 4 illustrates a schematic block diagramof an exemplary SPS receiver having a real-time clock that controlsoperation of the GPS receiver synchronously with expected arrival timesof information.

[0030]FIG. 5 is an exemplary multi-frame signal, for example, a GPSnavigation message, although the message could be any other signal. GPSsignals are transmitted on predictable schedules, and thus uponacquiring a GPS signal, the GPS receiver may synchronize its operationto receive only information of interest in the signal. In FIG. 5, forexample, the receive function of the GPS receiver is powered OFF duringthe arrival of frames SF1-SF3, and the receive function of the GPSreceiver is power ON during the arrival of frames SF4 and SF5 to permitreception and demodulation of the desired data almanac. The almanac datais known to occupy only sub frames 4 and 5, and not occupy subframes 1through 3. The almanac data is commutated over 25 sequential sets ofsubframes 4 and 5 in order to broadcast the almanac data. The datastructure shown in FIG. 5 begins synchronously with every 30-secondepoch from the start of the week. Thus, the time of the 1^(st) bit ofsubframe 1 is always transmitted some integer number times 30 secondssince the beginning of the week (n*30 sec). Likewise, the 1st bit ofsubframe 2 is known to begin 6 seconds later, or at time T=n*30+6seconds, since the start of the week. Subframe 3 begins at T=n*30+12seconds, subframe 4 begins at T=n*30+18 seconds, and subframe 5 beginsat T=n*30+24 seconds since the beginning of the week. Consequently, ifthe GPS receiver real-time clock is previously synchronized withrelatively accurate time, it can be programmed to power on at thebeginning of subframe 4 and power-off at the end of subframe 5,demodulating only bits for those subframes, and acquiring fresh almanacdata while minimizing its power consumption. 25 sets of subframes 4 and5 are required to obtain a complete issue of almanac data. In oneembodiment, the GPS receiver receives navigation information whiletracking a satellite.

[0031] In one embodiment, the GPS receiver is operated synchronouslywith an expected arrival of ephemeris and/or almanac information in aGPS navigation message transmitted by a GPS satellite or other source,like a repeater. In other embodiments, the GPS receiver operatessynchronously with an expected time of arrival of clock correctioninformation, ionospheric correction information, tropospheric correctioninformation, universal time coordinate offset correction information, orother having a scheduled time of arrival.

[0032] Generally, the GPS receiver may perform other operations duringtime periods when the receive function disabled, for example, during thearrival of frames SF1-SF3 in FIG. 5. More particularly, during periodswhen the GPS receiver is not receiving, the GPS receiver may operate toprocess signals received previously.

[0033] Navstar document ICD-GPS-200, Revision C, updated Oct. 11, 1999,which is hereby incorporated by reference in its entirety, list on pp.87 and 96 the ephemeris parameters and on page 108 the almanacparameters.

[0034] Satellite almanac data is useful in computing acquisitionassistance information, such as satellite Doppler and code phaseestimates and satellite visibility estimates as a function of time. Thealmanac data is tailored or optimized for a particular epoch time. Theepoch time is identified by a parameter TOA (time of almanac), which isthe epoch time described as the number of seconds into the week, and analmanac week number WNA. The GPS time clock and week numbers began atweek number zero and time zero on Jan. 5, 1980, week numbers incrementone count each week, rolling over after 1024 weeks, while the GPS timeclock increments one second each second, clearing at the start of thenext week. 604800 seconds are accumulated each week. An almanac weeknumber WNA and TOA identify precisely the reference time for the almanacwith an ambiguity of the 256-week period of the almanac week number. Thealmanac week number is 8 bits of the 10 bit GPS week number producing a256 week repeat time. The almanac equations for determining satelliteposition vs. time are driven by a time difference, that being the timein seconds between the current time and the TOA, accounting for weeknumber differences as well. A notation for how this is accomplished withalmanac is shown in Eqn. (1), in which a function translates the almanacinto a 3 dimensional satellite XYZ position vector in theearth-centered-earth-fixed (ECEF) coordinate frame as a function of time“t”, week number “wk”, for a particular satellite “sv”, and where thefunction SVPOS_alm uses the almanac satellite position equationsdescribed earlier.

SVPosXYZ_alm[sv]=SVPOS_alm(t, wk, sv)  (1)

[0035] The 3-dimensional vector SatPosXYZ_alm can be written asSVPosXYZ_alm=[SVPosX, SVPosY, SVPosZ], where the SVPosX component is theX-axis element, SVPosY is the Y-axis element, and SVPosZ is the Z-axiselement. The almanac equations translate time and week number intosatellite velocity vector as shown below in Eqn. (2).

SVVelXYZ_alm[sv]=SVVEL_alm(t, wk, sv)  (2)

[0036] The returned velocity vector is a 3-dimensional vector in theECEF Cartesian coordinate system indicating the satellite velocity atthe instantaneous time epoch “t”.

[0037] The satellite ephemeris data is also targeted and optimized for aparticular epoch time, called time of ephemeris, or TOE. The ephemerisdata is useful for computing precise satellite position data accurate towithin a few meters, provided the time difference between the currenttime “t” and TOE is within +/−2 hours under normal conditions. When thedifference between time t and TOE is within the range −7200 seconds<=t-TOE<=+7200 seconds, the ephemeris data reliably computes thesatellite position vector and velocity vector data to a precisionnecessary for user position and velocity computation. When time isoutside the range −7200 seconds <=t-TOE <=+7200 seconds, the satelliteposition accuracy is degraded and ephemeris data is not generally usefulfor autonomous position solutions.

[0038] Similar to almanac data, aged or inaccurate ephemeris data isuseful for computing acquisition assistance information, such as Dopplerand code phase estimates and satellite visibility estimates as afunction of time. The larger the time difference t-TOE the larger theerror in satellite position and velocity coordinates. Equationsdescribing ephemeris developed position and velocity data are shownbelow in Eqns. (3) & (4).

SVPosXYZ_eph[sv]=SVPOS_eph(t, wk, sv)  (3)

SVVelXYZ_eph[sv]=SVVEL_eph(t, wk, sv)  (4)

[0039] The satellite position and velocity equations used are asdescribed in ICD-GPS-200 document, Table 20-IV.

[0040] In one embodiment, the satellite position derived from ephemerisdata is compared to that derived from almanac data, for the samesatellite and at the same time “t”. For example, the 3-dimensionaldifference vector represented below by Eqn. (5)

ΔPXYZ[sv]=|SVPOS_eph(t, wk, sv)−SVPOS_alm(t, wk, sv)|  (5)

[0041] represents the linear range difference between thealmanac-derived position and ephemeris-derived position, where themagnitude operator |.| is a shortcut notation for the square root of thesum of the squares of each of the three vector difference elements.

[0042]FIG. 8 illustrates the growth of the difference in satelliteposition vectors described in Eqn. (5) as a function of the age ofephemeris and almanac data. The x-axis in FIG. 8 represents the age ofthe almanac/ephemeris in days, while the Y-axis represents the satelliteposition vector difference, i.e., Eqn. (5), for all satellites in theGPS constellation at a particular epoch time. At day number zero, afresh almanac and ephemeris for the entire GPS constellation wascollected. When the almanac and ephemeris are fresh (day number 0, 1,2), the difference in satellite positions is relatively small, on theorder of 1-2 km. As the data sets age, the difference in satelliteposition derived from ephemeris as compared to almanac grows to be nomore than 300 km after about 100 days. Most of the difference is due toerror growth in satellite positions from the ephemeris data, not fromthe almanac data, because of the inclusion of the amplitude of sin andcosine corrections to arguments of latitude, orbital radius andinclination angle. Compared to the user-to-satellite range, which isbetween 20,100 km and 28,500 km, a 300 km position error in thesatellite position vector is very small compared to the geometry of theuser to satellite range (1/67^(th) to 1/95^(th) of the range vector).Thus, satellite visibility equations that describe the azimuth andelevation of a particular satellite as a function of time will be inerror by no more than approximately 1-2 degrees.

[0043]FIG. 9 shows the satellite velocity vector difference betweenephemeris and almanac data as both age from their “fresh” state (day=0,1, 2) to the relatively old 100 days. Most of the satellite velocityvector difference is in the along-track direction, but it could be inthe direction of the user-to-satellite unit vector which means ittranslates directly to predicted Doppler error. After about 100 days,the worst-case velocity vector difference is less than 35 meters persecond, which translates to about 183 Hz predicted Doppler error if allthe velocity error is in the direction of the user-to-satellite unitvector; a low probability case. Most of the difference shown in FIG. 9is due to the aging ephemeris data, not the aging almanac data. Thepredicted Doppler with 100 day-old ephemeris data is sufficient toacquire satellites if the acquisition algorithm takes into account thenearly linear growth in Doppler uncertainty due to the aging ephemerisdata. For example, the acquisition algorithm could modify the Doppleruncertainty as a function of approximately ephemeris age, for example,Du=183 Hz*Days/100, and expand the Doppler search space accordinglybased on the age of the stored ephemeris data as it ages.

[0044]FIGS. 10 and 11 illustrate the satellite position and velocitydifference relationship of a current ephemeris data (day=0) compared toalmanac data that was current 80 days earlier. At t=−80 days, freshalmanac data is captured and stored in memory. At t=0 days, freshephemeris data is collected and compared to the older almanac data.After the ephemeris data is collected, one can plot theposition/velocity difference backwards in time, i.e., from T=−80 days(almanac fresh, ephemeris −80 days old) to T=0 days (ephemeris fresh,almanac 80 days old). This allows a direct comparison of the performanceof almanac data after 80 days of aging when the almanac positions andvelocities are compared to ephemeris positions and velocities when timeT is within the accurate ephemeris time period of −7200 seconds<=t-TOE<=+7200 seconds. At day zero (x-axis), ephemeris data is mostaccurate and the aging almanac is the source of most of theposition/velocity difference. FIG. 10 indicates that even after 80 daysof aging, almanac data still returns position data within 40 km, andvelocity accuracy within about 5 meters per second (FIG. 11) of truth,truth data being determined from the fresh ephemeris data.

[0045] In FIGS. 10 and 11, plots 1000, 1002, 1004 & 1006 represent errorversus time for a group of four satellites, whose position and velocityerrors are substantially greater than the other 24 satellites in theconstellation. Periodically, satellites in the GPS constellation arere-phased or moved in the orbit to re-align the orbit for more optimumcoverage. As a result, in FIGS. 10 and 11, satellites corresponding toplots 1000, 1002, 1004 & 1006 have been re-phased in orbit some time inthe 80 days between acquisition of the almanac and ephemeris data. Thusthe old orbit was captured by the “old” almanac data, while the “new”orbit was captured by the “new” ephemeris data. Consequently, there is asubstantially large position and velocity error in re-phased satelliteposition and velocity comparisons between almanac and ephemeris data.This error is detectable by much larger than normal error growth overtime, and the fact that a particular satellite has been re-phased inorbit can also be detected by a much larger error than expected in theold almanac satellite position/velocity data compared to the newerephemeris satellite position/velocity data. The almanac data that isstored should probably be replaced with fresh almanac, or simply use thenewly gathered ephemeris data to acquire satellites. Attempts to use theold “pre-phasing” almanac or ephemeris data to acquire a satellite“post-rephasing” will likely result in failure to detect the satellitedue to a large estimated Doppler error.

[0046] According to another aspect of the disclosure, an SPS receiverdownloads ephemeris, during normal usage, via a cellular networkover-the-air protocol message or directly from GPS satellites in orderto compute accurate position solutions at the SPS receiver. When newephemeris is obtained, the SPS receiver compares the accuracy of thepreviously stored almanac data to the fresh ephemeris data, anddepending on an error threshold, decides whether to replace thesatellite's almanac data with the ephemeris data or collect freshalmanac directly from satellites.

[0047] In some embodiments, the SPS receiver, which may be embedded in acommunications device, stores both almanac and ephemeris data for eachsatellite and compares the accuracy of the stored almanac data with thestored fresher ephemeris data, and decides to use either the almanacdata or the ephemeris data for each satellite acquisition assistcomputation dependent on the inaccuracy or error and/or age of almanacand ephemeris data. In another embodiment, the SPS receiver storesalmanac and ephemeris data for each satellite in the constellation, andcomputes assist data from the most accurate or fresh source, eitheralmanac or ephemeris data. A failure to detect a particular satelliteusing the assist data will trigger a request for fresh ephemeris for thenon-acquired satellite from a wireless network. In another embodiment,the SPS receiver stores ephemeris data for generation of satelliteacquisition assist at times outside the −7200 second <=t-TOE<=+7200 timeperiod. When the expected error in the assist data is greater than athreshold, the wireless handset requests fresh ephemeris for theparticular satellite. In still another embodiment, the SPS receiverstores ephemeris data for generation of satellite acquisition assist attimes outside the −7200 second <=t-TOE<=+7200 time period. When the ageof ephemeris exceeds a particular threshold after which the assist databecomes inaccurate, the GPS receiver embedded in the communicationsdevice requests fresh ephemeris for the particular satellite.

[0048] According to another aspect of the disclosure, a satellitepositioning system receiver not attempting to acquire satellites, upondetermining that ephemeris data for at least one satellite stored on thesatellite positioning system receiver is no longer useful for generatingsatellite acquisition assistance data, the SPS receiver periodicallyupdates ephemeris data, for example, from a communications network ordirectly from the satellites, while the satellite positioning systemreceiver is not attempting to acquire satellites until the satellitepositioning system receiver has received updated ephemeris data for theat least one satellite. In one embodiment, ephemeris data is updatedwhile the satellite positioning system receiver is not attempting toacquire satellites until the satellite positioning system receiver hasreceived updated ephemeris data for all satellites.

[0049] According to another aspect of the disclosure, upon determiningthat ephemeris data stored on the satellite positioning system receiveris outdated, a satellite positioning system receiver determines that aparticular satellite is visible using the outdated ephemeris data whilenot attempting to acquire satellites with the satellite positioningsystem receiver, and requests current ephemeris data for the samesatellite with a over-the-air message while not attempting to acquiresatellites with the satellite positioning system receiver. According toanother aspect of the disclosure, a satellite positioning systemreceiver determines that ephemeris data is too inaccurate to acquire asatellite by attempting to acquire the satellite using the storedephemeris data. If the ephemeris data is inaccurate, accurate ephemerisis requested in an over-the-air message.

[0050] According to another aspect of the disclosure, low-resolutionsatellite orbital information is generated from information obtainedfrom at least one issue of ephemeris data. In some embodiments, thelow-resolution satellite orbital information derived from informationobtained from the at least one ephemeris issue has a resolution levelsufficient for computing satellite location and velocity information.Satellite position and velocity information may be used to determinesatellite Doppler estimates and uncertainty ranges, which may be usefulfor initial satellite acquisition by SPS receivers. In otherembodiments, the low-resolution satellite orbital information derivedfrom the at least one issue of ephemeris data is substantially the sameas almanac data. Thus the low-resolution satellite orbital informationis useful for SPS receivers not having previously stored almanac data,and in receivers where previously stored almanac data becomes lost,corrupted, or outdated. This process may also eliminate the need toobtain almanac data directly from the satellites or from an over-the-airmessage. In some SPS receivers it is impractical for receivermanufacturers to store almanac data or to store timely almanac data onthe receiver. In these and other instances it is desirable to generatelow-resolution satellite orbital information on SPS receivers.

[0051] Generally, a good approximation can be made to the almanacparameters from an ephemeris data set, at least for satelliteacquisition purposes, by using the ephemeris data without regard to theamplitude of sin and cosine corrections to arguments of latitude,orbital radius, and inclination angle and limiting the orbit computationto the following original ephemeris parameters:

[0052] M₀—Mean Anomaly at Reference Time;

[0053] e—Eccentricity;

[0054] (A)^(1/2)—Square Root of the Semi-Major Axis;

[0055] (OMEGA)₀—Longitude of Ascending Node;

[0056] i₀—Inclination Angle at Reference Time;

[0057] ω—Argument of Perigee;

[0058] OMEGADOT—Rate of Right Ascension; and

[0059] IDOT—Rate of Inclination Angle. It is generally acceptable to useephemeris data for satellite acquisition purposes during periods of timesubstantially outside the +/−2-hour interval, for example, 100 or moredays.

[0060]FIG. 6 illustrates a process for generating relativelylow-resolution satellite orbital information from at least one issue ofephemeris data from the same satellite. The SPS receiver obtainsmultiple issues of ephemeris data EPHI 601, EPH2 602, EPH3 603 and EPH4604, etc., from the same satellite from time to time, for example, inconnection with SPS receiver position solutions. The ephemeris data maybe acquired directly from SPS satellites or it may be requested fromsome other source, for example, from an assisted base-station via anover-the-air message. The interval between sequential issues ofephemeris data may be weeks or months, depending upon the resolutionrequired of the low-resolution satellite orbital information derivedtherefrom, although longer or shorter time intervals may be usedalternatively. The intervals between ephemeris data issues preferablyexceed the valid time period of any particular ephemeris issue, e.g.,TOE +/−2 hours.

[0061] The multiple issues of ephemeris data are obtained during thenormal course of SPS receiver operation, for example, when required fordetermining a position or location fix of the receiver. Thus, generally,it is unnecessary to allocate SPS receiver resources specifically toobtaining ephemeris data for the sole purpose of generatinglow-resolution satellite orbital information, since the ephemerisinformation is generally acquired for other purposes. In some instances,however, it may be desirable to obtain ephemeris data specifically foruse in generating or updating the resolution of the low-resolutionsatellite orbital information, for example, to ensure that the derivedlow-resolution satellite orbital information has the desired resolution.

[0062] In FIG. 6, ephemeris based satellite position and velocity iscalculated at block 610. The satellite position vectors(SVPosXYZ_eph[sv]) and satellite velocity vectors (SVVelXYZ_eph[sv]) areartifacts of SPS position determinations at particular time epoch basedon the corresponding ephemeris data. Preferably, for each ephemeris dataset, at least one satellite position and velocity vector coordinate pairis stored in a database on the SPS receiver, as indicated at block 612.More particularly, the parameters stored include the satellite positionvector SVPosXYZ_eph[i] , satellite velocity vector SVVelXYZ_eph[i],satellite identification (SVID[i]), the time associated with thesatellite position/velocity data (TOW[i]), the GPS week number (Wn[i]),and optionally the time of ephemeris (TOE[i]). The index [i] indicatesthe entry number in the database for the particular parameter, example;svid[i] the corresponding satellite ID for that entry. The TOE[I] may bestored instead of TOW provided that satellite position and velocity arecomputed at TOE time instead of TOW time. It is not likely that thenormal position computation function would actually compute time atexactly TOE time, so it is more practical to store TOW time associatedwith the time of computation of the satellite position/velocity data. Inapplications in which no additional calls of the ephemeris basedsatellite position and velocity function are used, the storage of TOEcan be avoided. The storage of TOW[i] may be avoided if additional callsof the ephemeris based satellite position and velocity function can betolerated, then it is easier to compute the position and velocitycoordinates at TOE time, which allows for simplification of the databasebecause TOW time would be stored as TOE time, which requires fewer bitssince it is an integer representation. If the SPS receiver is useddaily, the logic has the luxury to store data at some periodic rate, forexample, weekly. If the usage pattern is much more sparse, say once peryear, then every ephemeris data set acquired would be used to update thestored almanac parameters.

[0063] In FIG. 6, at block 614, the satellite orbital information isobtained by a direct curve-fit function that forms a satellite positioncurve and satellite velocity curve from the plurality of satelliteposition vectors and velocity vectors as a function of time. Computationof the Keplerian orbit elements may be determined in the traditionalway. An example of computing Keplerian orbital elements from satelliteposition and velocity data points is discussed starting on page 61 in“Fundamentals of Astrodynamics”, by Bate, Mueller, and White, publishedby Dover Publications, 1971. The satellite orbital information is storedat block 618 and is used later for acquisition assist generation.

[0064] In some embodiments, portions of the corresponding plurality ofissues of ephemeris data received for the at least one satellite, forexample, eliminating sine and cosine harmonic terms in order to removethis long-term error source in the ephemeris orbit equations. This canbe accomplished by setting the amplitude of sin and cosine correctionsto arguments of latitude, orbital radius, and inclination angle to zero

[0065] In some embodiments, the satellite orbital coefficientsdetermined at block 614 are converted to almanac data resolution andformat for convenience. It is not necessary to scale the determinedsatellite orbital coefficients into the same number of bits and scalefactor transmitted by GPS satellites. Scaling allows almanac dataobtained directly from GPS satellites or the satellite orbitalcoefficients to be stored in the same holding register by converting thesatellite orbital coefficients into the format and resolution of almanacdata. Also, the precision of the plurality of issues of ephemeris datamay be reduced to a resolution level comparable with almanac data forthe same satellite as described in U.S. Pat. No. 6,437,735.

[0066] According to another aspect of the disclosure, almanac datastored on the SPS receiver is updated occasionally based upondifferences in ephemeris-based satellite position and velocityinformation and almanac-based satellite position and velocityinformation. This strategy eliminates the necessity of downloadingupdated versions of almanac data.

[0067] The almanac data may be initially stored in memory on the SPSreceiver during manufacture, for example, via a serial port connectionprior to shipping from the factory. Alternately, almanac data could beinstalled in the SPS receiver when it is initially delivered to theuser, for example, upon activation of a SPS enabled cellular telephone.The almanac data may also be obtained directly from SPS satellites, forexample, using the synchronization scheme discussed above, or byconventional means requiring at least 12.5 minutes of continuoussatellite tracking, which can substantially drain the handset battery ifnot coupled to an external power source. According to this aspect of thedisclosure, however, it is only be necessary to obtain the almanac dataonce, regardless of the acquisition means.

[0068] This scheme takes advantage of the fact that the SPS receiveroccasionally acquires fresh ephemeris data for receiver positioncomputations. The ephemeris data may be acquired directly from SPSsatellites in about 30 seconds of continuous tracking, or it can berequested via an over-the-air message set from an assisted SPS basestation or from some other source.

[0069] In FIG. 7, the SPS receiver obtains multiple issues of ephemerisdata EPH1 701, EPH2 702, EPH3 703 and EPH4 704, etc., from the samesatellite from time to time, for example, in connection with SPSreceiver position solutions, separated by some time interval asdiscussed above in connection with FIG. 6. As discussed, an artifact ofposition computations is satellite position vector (SVPosXYZ_eph[i]) andsatellite velocity vector (SVVelXYZ_eph[i]) information for eachsatellite at a particular time epoch based on the fresh ephemeris data.In FIG. 7, satellite position vector SVPosXYZ_eph[i], satellite velocityvector SVVelXYZ_eph[i], satellite identification (SVID), time associatedwith the satellite position/velocity data (TOW[i]), GPS week number(Wn[i]), and optionally the time of ephemeris (TOE[i]) are stored atblock 712, as discussed above in relative to FIG. 6.

[0070] The ephemeris data creates relatively true satellite position andvelocity vectors during a portion of time bracketing the time ofephemeris (TOE) by two hours, i.e., TOE +/−2 hours. Thus any satelliteposition/velocity data derived from fresh ephemeris data at a time epochbetween or within the range of TOE +/−2 hours can be used as a “truthmodel” when compared to almanac-derived satellite position and velocityvector data for the same time epoch. As the stored almanac data ages,the error between the ephemeris and almanac derived position andvelocity vectors grows to some unacceptable limit. The differences, alsoreferred to as position and velocity residuals, can be used to computeadjustments to the originally stored almanac parameters to reduce thealmanac produced errors.

[0071] The process generally compares almanac derived satellite positionand velocity information to satellite position and velocity informationderived from current ephemeris data, and derives corrections for currentalmanac parameters based upon the comparison. The corrections are usedto update the almanac parameters (new_param=old_param+correction), forwhich the new parameters are stored for future acquisitions, forexample, acquisitions outside the window of applicability of the currentephemeris data.

[0072] Upon development of a database of several current and pastsatellite position coordinates for several satellites, almanac dataerror may be measured. In FIG. 7, the almanac data stored at block 720and the SVID[i], TOW[i], and Wn[i] parameters stored at block 712 areused to compute almanac-based satellite position SVPosXYZ_alm[i] andvelocity SVVelXYZ_alm[i] coordinates using an almanac-based satelliteposition and velocity calculator 722. The almanac position and velocityinformation is computed at the time indicated by TOW[i] for eachsatellite (SVID[i]), which is obtained from block 712. The almanac-basedcomputation results are stored at block 724.

[0073] In FIG. 7, at block 730, differences in satellite position and insatellite velocity vectors are computed. Specifically,APXYZ[i]=SVPOS_eph(t, wk, sv)−SVPOS_alm(t, wk, sv)represents the 3dimensional difference vector in position based on current satelliteephemeris and the aging almanac position, and ΔVXYZ[i]=SVVel_eph(t, wk,sv)−SVVel_alm(t, wk, sv) represents the 3-dimensional difference invelocity based on current satellite ephemeris and the aging almanacvelocity. The residuals are computed and stored in a database for eachsatellite stored in database at steps 712 and 724.

[0074] The position and velocity residual information may be used as anerror signal, which may be used to correct the aging almanac data. InFIG. 7, at block 734, parameters of the original almanac are adjusted bya function based on the size of the residuals over the time intervalcorresponding to the samples in the truth model database. After eachadjustment of the almanac orbit parameters, the process can repeat,creating a new set of residuals for each satellite.

[0075] In one embodiment, the process uses Least-Squares (LS)computations, or it may be performed iteratively to minimize the numberof iterations. If a LS computation approach is used, the problem is bestsolved through computations and partial derivatives of the satellite andposition error vectors with respect to the orbital parameters, i.e.,modeling first order, underlying sensitivities involved. It can also beperformed by testing sensitivities of each almanac parameter and byadjusting each one dependent on the direction of the dominant error inthe residuals.

[0076] For example, if most of the error is in the along-trackdirection, then the mean-motion parameter should be adjusted to minimizethe along track error on subsequent iterations. The essentially linearerror growth in along-track position error can be attributed largely toa misrepresentation of a single orbital element, namely, the mean motionof the satellite. This parameter represents the linear angular rate ofgrowth of the projected path of the satellite in a circle whichcircumscribes the ellipse (along which its actual motion occurs) and isrelated to two parameters within the ephemeris, the semi-major axis, a,and the correction to mean motion, An, both appearing in the Equationsbelow:

M=n(t−t _(p))  (6)

n={square root}μ/a ³ +Δn  (7)

[0077] where “t” denotes time, “tp” denotes the time of perigee passage,“n” is the mean motion, and “M” is the angle within the circumscribingcircle, and where “μ” represents a gravitational constant.

[0078] Given a measurement of the along-track error component of thealmanac by comparison with a current ephemeris, a correction can begenerated by making an adjustment to the mean motion parameter assumedby the almanac, thereby reducing its dominant error component. Otherparameters can be also adjusted depending on the direction and size ofthe residual error vectors.

[0079] As the process continues to refresh the stored almanac data, theolder ephemeris based data stored in memory are replaced with newerposition and velocity information so that the process of measuringerrors are based mostly on newer ephemeris data collected in the futurerelative to the date of the almanac data. The process thus measures theerror growth of the stored almanac as it ages compared to the truthephemeris. Some minimum number of stored data points per satellite needsto be collected in memory.

[0080] In some instances, it may not be possible to update the almanacdata, for example, in cases where the residuals are not sufficientlyreduced by iteration. In these instances is may be necessary to acquirenew almanac data or to generate low-resolution satellite informationfrom ephemeris data as discussed above.

[0081] Situations that may result in convergence of the residualsinclude Department of Defense (DoD) satellite orbit changes (i.e., orbitre-phasing) within the time frame of the satellite position and velocityhistory data stored in database, for example, between the time of EPH2and EPH3. Since the time frame between the collection of EPH2 and EPH3position and velocity information may be relatively long, weeks ormonths, there is no sure way to know whether a satellite has beenre-orbited. In one embodiment, ephemeris data parameters that indicatewhether a satellite trajectory has been changed are stored. Examples ofparameters that may be stored to detect significant changes in satelliteorbits, such as may occur during the re-orbiting of a spacecraft,include any one or more of the following ephemeris parameters, amongothers, for detecting significant changes in satellite orbit:

[0082] M₀—Mean Anomaly at Reference Time;

[0083] e—Eccentricity;

[0084] (A)^(1/2)—Square Root of the Semi-Major Axis;

[0085] (OMEGA)₀—Longitude of Ascending Node;

[0086] i₀—Inclination Angle at Reference Time;

[0087] ω—Argument of Perigee;

[0088] OMEGADOT—Rate of Right Ascension; and

[0089] IDOT—Rate of Inclination Angle.

[0090] For each of these parameters, an expected guard band would becreated, basically a minimum and a maximum value that would bracket theexpected next value of the parameter given that no re-orbit eventoccurred. When a next ephemeris data set is acquired, for example, fromthe cellular network, each new parameter would be tested against itsexpected guard-banded range based on the previous history of ephemerisdata. If any of the above parameters exceeded its expected maximum orminimum value, then it's likely that a re-orbit operation occurred sincethe last ephemeris set was observed for this particular satellite. Underthese circumstances, it would be necessary to replace any storedsatellite position and velocity data derived from the ephemeris datafrom the re-orbited satellite.

[0091] A particular handset must be used periodically for positionfixing in order for the handset to obtain current copies of allephemeris data for acquired or visible satellites in order to compute anaccurate position solution internally. Thus, depending on the usagepattern of a particular handset, it may be used frequently enough toupdate the stored almanac or low-resolution satellite orbit parameters,or it may be used so infrequently that the almanac or low-resolutionsatellite orbit parameters get stale. Some method of updating thealmanac data or low-resolution satellite orbit parameters in low usagepattern handsets is required.

[0092] The satellites in the GPS constellation are in an approximately12-hour periodic orbit that precesses about 4 minutes per day. Thismeans that the same satellite appears at the same point of the sky 23hours and 56 minutes later (not 24 hours). Each satellite in theconstellation rises and sets during different parts of the day. Ahandset that is used one time per week, say at 8 AM local time, willobtain fresh ephemeris for the satellites visible at that time, but notobtain ephemeris for other satellites in the constellation. This isbecause the over-the-air protocol messages in cellular AGPS assisttransport fresh ephemeris data to the handset only for the satellitesthat are currently visible at the user's approximate location. Since theconstellation of visible satellites is not much different at 8 AMbetween adjacent weeks, the handset will only obtain fresh ephemeris forthe same satellites over and over again until the constellation slowlyrotates relative to the user's local clock. Consequently, it can takemany months for other satellites in the constellation to become visibleto the handset because they are not visible to the handset at 8 AM localtime, and will not be visible at that time for many months.Consequently, it is possible for the method disclosed herein to update acertain number of satellites frequently, and not update other satellitesfor a long time because the satellites not updated are on a differentvisibility schedule relative to the user's usage pattern.

[0093] One method to counter this aging rate difference is to programthe handset to recognize when certain satellites stored almanac or lowresolution satellite orbit parameters are getting old or stale, eitherbecause the handset is not being used for periodic positioncomputations, or because the usage pattern for position computation issuch that certain satellites are never updated. The handset can beprogrammed to determine when the satellites needing update are visible,by normal computation of satellite visibility using the aging almanac orlow-resolution satellite orbit parameters. In one embodiment, thehandset would not attempt to acquire the satellite, only recognize thatthe satellite with the aging almanac or low resolution satellite orbitparameters is presently visible using local time from a real-time clockor cellular over-the-air message, the handset's last known location or acellular over-the-air message intended for transporting approximateposition to the handset, and the aging almanac or low resolutionsatellite orbit parameters. Using this data, the handset can then knowwhen the satellite for which the almanac or low resolution satelliteorbit parameters are in need of update, and then request an ephemerisupdate for all satellites visible at that time. The handset would notnecessarily have to compute position, but still it can go through theover-the-air protocol exchange as if it was attempting a position fix.When the cellular network receives the request for fresh ephemeris data,all ephemeris data for satellites visible at that time, including thesatellite needing an update to its stored almanac or low-resolutionsatellite orbit parameters, would be transported to the handset. Thehandset could proceed to a position fix, or simply use the algorithmsdescribed in this disclosure to update the stored almanac orlow-resolution satellite orbit parameters to be used for satelliteacquisition assist if and when the handset or user needs to acquiresatellites and produce a position fix. This update procedure would beaccomplished without ever turning on the handset's internal GPSreceiver, the update process could be scheduled at times of the day whenlittle cellular over-the-air traffic was occurring (for example, 2 am),so that fresh almanac or low resolution satellite orbit parameters arealways available for every satellite in the constellation.

[0094] While the present disclosure and what are considered presently tobe the best modes of the inventions have been described in a manner thatestablishes possession thereof by the inventors and that enables thoseof ordinary skill in the art to make and use the inventions, it will beunderstood and appreciated that there are many equivalents to theexemplary embodiments disclosed herein and that myriad modifications andvariations may be made thereto without departing from the scope andspirit of the inventions, which are to be limited not by the exemplaryembodiments but by the appended claims.

What is claimed is:
 1. A method in a satellite positioning systemreceiver, comprising: receiving a plurality of issues of ephemeris datafor at least one satellite; deriving relatively low-resolution satelliteorbital information for the at least one satellite from satelliteinformation obtained from the corresponding plurality of issues ofephemeris data received for the at least one satellite.
 2. The method ofclaim 1, deriving the satellite orbital information for the at least onesatellite includes obtaining average satellite orbital coefficients fromthe corresponding plurality of issues of ephemeris data and reducing theresolution of the satellite orbital coefficients obtained.
 3. The methodof claim 1, deriving the satellite orbital information for the at leastone satellite includes reducing the precision of satellite informationobtained from the corresponding plurality of issues of ephemeris data toa resolution level comparable with almanac data for the same satellite.4. The method of claim 1, deriving the satellite orbital information forthe at least one satellite includes eliminating portions of thecorresponding plurality of issues of ephemeris data received for the atleast one satellite.
 5. The method of claim 4, eliminating portions ofthe corresponding plurality of issues of ephemeris data receivedincludes eliminating at least one of sine and cosine harmonic terms. 6.The method of claim 1, deriving the satellite orbital informationincludes forming a plurality of estimated satellite locations for the atleast one satellite based upon the plurality of issues of ephemeris datafor the corresponding satellite, and computing satellite orbitalcoefficients for the at least one satellite based upon the estimatedsatellite locations.
 7. The method of claim 6, converting the satelliteorbital coefficients to almanac data resolution and format.
 8. Themethod of claim 1, obtaining satellite positioning and velocityinformation from each of the plurality of issues of ephemeris data;storing the satellite positioning and velocity information on thesatellite positioning system receiver; deriving relativelylow-resolution satellite orbital information for the at least onesatellite from satellite location and velocity information obtained fromthe corresponding plurality of issues of ephemeris data.
 9. The methodof claim 1, determining location and velocity information for the atleast one satellite from the corresponding satellite orbital informationderived.
 10. The method of claim 9, determining a Doppler estimate anduncertainty range for the at least one satellite from the correspondingsatellite location and velocity information.
 11. The method of claim 1,updating the satellite orbital information for the at least onesatellite with updated ephemeris data.
 12. The method of claim 11,obtaining updated ephemeris data if updated ephemeris data is not storedon the satellite positioning system receiver.
 13. A method in asatellite positioning system receiver having stored almanac data, themethod comprising: determining information for a satellite usingephemeris data; determining information for the same satellite using thestored almanac data; determining an error between the satelliteinformation determined from the ephemeris data and the satelliteinformation determined from the stored almanac data; updating the storedalmanac data based upon the error.
 14. The method of claim 13,determining satellite information for the same satellite includesdetermining satellite location and velocity information for the samesatellite using the ephemeris data and using the stored almanac data;determining the error includes determining an error between thesatellite location and velocity information determined from theephemeris data and from the stored almanac data.
 15. The method of claim14, determining satellite location and velocity information for the samesatellite using the ephemeris data and using the stored almanac dataduring a common epoch.
 16. The method of claim 14, determining satellitelocation and velocity information for the same satellite using theephemeris data and using the stored almanac data within a specified timeinterval of Time of Ephemeris (TOE) for the ephemeris data.
 17. Themethod of claim 14, determining satellite information for the samesatellite includes determining satellite location and velocityinformation for the same satellite using the ephemeris data and usingthe updated almanac data; determining the error includes determining arevised error between the satellite location and velocity informationdetermined from the ephemeris data and from the updated almanac data;updating the updated almanac data based upon the revised error.
 18. Amethod in a satellite positioning system receiver having stored almanacdata, the method comprising: determining, at corresponding time periods,location and velocity information for a satellite based on a pluralityof issues of ephemeris data for the satellite; determining location andvelocity information for the satellite based on the stored almanac datafor the satellite at the same time periods for which the location andvelocity information based on the plurality of issues of ephemeris datawas determined; for each time period, determining error between thelocation and velocity information for the satellite based on theephemeris data and the location and velocity information for thesatellite based on the stored almanac data; updating the stored almanacdata based upon the error.
 19. A method in a satellite positioningsystem receiver having a battery, the method comprising: determiningwhether the receiver is connected to a power supply other than itsbattery; beginning continuous reception of satellite positioning systemnavigation data when the receiver is connected to a power supply otherthan its battery; storing the navigation data received in memory of thereceiver.
 20. The method of claim 19, discontinuing reception of thesatellite positioning system navigation data if the receiver isdisconnected from the power supply other than its battery.
 21. Themethod of claim 19, continuing reception of the satellite positioningsystem navigation data until reception of the navigation data iscomplete if the receiver is disconnected from the power supply otherthan its battery during reception of the navigation data.
 22. The methodof claim 21, continuing reception of the satellite positioning systemnavigation data if the receiver is disconnected from its power supplyother than its battery only if a predetermined portion of the navigationdata has already been received.
 23. The method of claim 19, receivingsatellite positioning system navigation data includes receiving almanacinformation directly from a satellite.
 24. The method of claim 19,receiving satellite positioning system navigation data includesreceiving ephemeris information directly from a satellite.
 25. A methodin a satellite positioning system receiver, the method comprising:operating the receiver synchronously with an expected time of arrival ofinformation from at least one satellite of a satellite positioningsystem; receiving the information from the at least one satellite whenthe receiver is operating during the expected time of arrival of theinformation.
 26. The method of claim 25, operating the receiversynchronously with an expected arrival of specific subframe informationfrom at least one satellite of a satellite positioning system, receivingthe specific subframe information when the receiver is operating. 28.The method of claim 25, synchronously disabling a receiver operation ofthe receiver during time periods when the arrival of the information isnot expected.
 29. The method of claim 27, operating the receiver duringtime periods when the arrival of information is not expected forperforming functions other than receiving the information.
 30. Themethod of claim 25, operating the receiver synchronously with anexpected arrival of at least one of ephemeris and almanac informationfrom at least one satellite of a satellite positioning system.
 31. Themethod of claim 25, operating the receiver synchronously with anexpected arrival of at least one of clock correction information,ionospheric correction information, tropospheric correction information,universal time coordinate offset correction information.
 32. The methodof claim 25, receiving navigation information from at least onesatellite of the satellite positioning system when acquiring a satelliteof the satellite positioning system.
 33. The method of claim 25,receiving navigation information from at least one satellite of thesatellite positioning system when tracking a satellite of the satellitepositioning system.
 34. A method in a satellite positioning systemreceiver not attempting to acquire satellites, the method comprising:determining that ephemeris data for at least one satellite stored on thesatellite positioning system receiver is no longer useful for generatingsatellite acquisition assistance data; periodically requesting updatedephemeris data for satellites from a communications network while thesatellite positioning system receiver is not attempting to acquiresatellites until the satellite positioning system receiver has receivedupdated ephemeris data for the at least one satellite.
 35. The method ofclaim 34, periodically requesting updated ephemeris data for satellitesfrom the communications network while the satellite positioning systemreceiver is not attempting to acquire satellites until the satellitepositioning system receiver has received updated ephemeris data for allsatellites.
 36. A method in a satellite positioning system receiver, themethod comprising: determining that ephemeris data stored on thesatellite positioning system receiver is outdated; requesting currentephemeris data for the same satellite with a over-the-air message whilenot attempting to acquire satellites with the satellite positioningsystem receiver.
 37. A method in a satellite positioning systemreceiver, the method comprising: attempting to acquire at least onesatellite with stored ephemeris data, determining that the ephemerisdata is too inaccurate to acquire the least one satellite; requestingaccurate ephemeris data for the same satellite in an over-the-airmessage.
 38. A method in an SPS received enabled communication devicehaving stored almanac data, the method comprising: storing ephemerisdata received from a cellular network; selecting whether to use thestored almanac data or the stored ephemeris data for determiningsatellite acquisition information; determining satellite acquisitioninformation using the selected almanac data or ephemeris data.
 39. Themethod of claim 38, selecting the almanac data or ephemeris data basedon the relative ages of the almanac and ephemeris data.
 40. The methodof claim 38, selecting the almanac data or the ephemeris data based onthe estimated accuracies of the almanac data and ephemeris data.